Often applied in industrial measurements technology, especially also in connection with the control and monitoring of automated manufacturing processes, for highly accurate ascertaining of densities of media, for example, liquids or gases, flowing in a process line, for example, a pipeline, are vibronic density measuring devices formed by means of a measuring device electronics (most often at least one microprocessor) as well as a measuring transducer of vibration-type electrically connected with the measuring device electronics and flowed-through during operation by the medium to be measured. Such density measuring devices—embodied, for example, as so-called four-conductor- or also as so-called two conductor devices—have been known for a long time, not least of all also in the form of Coriolis mass flow-/density measuring devices or also in the form of viscosity-density measuring devices, and are established in industrial applications. Examples of such vibronic density measuring devices, respectively suitable measuring transducers, are described, among other things, in US-A 2004/0123645, US-A 2006/0096390, US-A 2007/0119264, US-A 2008/0047362, US-A 2008/0190195, US-A 2010/0005887, US-A 2010/0011882, US-A 2010/0257943, US-A 2011/0161017, US-A 2011/0219872, US-A 2011/0265580, US-A 2012/0123705, U.S. Pat. Nos. 4,491,009, 4,524,610, 4,801,897, 4,996,871, 5,024,104, 5,287,754, 5,291,792, 5,349,872, 5,531,126, 5,705,754, 5,796,010, 5,796,011, 5,831,178, 5,945,609, 5,965,824, 6,006,609, 6,092,429, 6,223,605, 6,311,136, 6,477,901, 6,513,393, 6,647,778, 6,666,098, 6,651,513, 6,711,958, 6,840,109, 6,920,798, 7,017,424, 7,059,176, 7,077,014, 7,200,503, 7,216,549, 7,325,462, 7,360,451, 7,792,646, Published International Applications, WO-A 00/34748, WO-A 01/02 816, WO-A 2008/059262, WO-A 2013/092104, WO-A 85/05677, WO-A 88/02853, WO-A 89/00679, WO-A 94/21999, WO-A 95/03528, the WO-A 95/16897, WO-A 95/29385, WO-A 98/02725, WO-A 99/40 394, WO-A 00/34748 or also in the not earlier published German patent applications DE102013101369.4, DE102013102708.3, respectively DE102013102711.3. The measuring transducer of each of the density measuring devices shown therein comprises at least one, at least sectionally straight and/or at least sectionally curved, e.g. U-, V-, S-, Z- or -shaped, measuring tube having a lumen surrounded by a tube wall and serving for guiding the medium, wherein the tube wall, depending on application, is typically made of a metal, for instance, titanium, respectively a titanium alloy, tantalum, respectively a tantalum alloy, zirconium, respectively a zirconium alloy, a stainless steel or a nickel based alloy, or, for example, also of silicon. A caliber of the measuring tube can lie, depending on application, typically in a range between 0.5 mm and 100 mm.
The at least one measuring tube of such a measuring transducer is adapted to guide medium in the lumen and during that to be caused to vibrate such that the at least one measuring tube executes wanted oscillations, namely mechanical oscillations about a resting position with a wanted frequency co-determined by the density of the medium and consequently usable as a measure for the density. In the case of conventional density measuring devices, typically bending oscillations at a natural resonant frequency serve as wanted oscillations, for example, such bending oscillations, which correspond to a natural bending oscillation, fundamental mode inherent to the measuring transducer. In such case, the oscillations of the measuring tube are resonant oscillations, which have exactly one oscillatory antinode. The wanted oscillations are in the case of an at least sectionally curved measuring tube additionally typically so embodied that the measuring tube moves in a pendulum-like manner about an imaginary oscillation axis imaginarily connecting an inlet-side end and an outlet-side end of the measuring tube in the manner of a cantilever clamped on one end, while, in contrast, in the case of measuring transducers with a straight measuring tube the wanted oscillations are most often bending oscillations in a single imaginary plane of oscillation. It is additionally known, at times, to excite the at least one measuring tube even to lasting oscillations outside of resonance for the purpose of performing repeated checks of the measuring transducer during operation of the density measuring device, as well as to evaluate the oscillations outside of resonance, for example, in order, such as described in the aforementioned US-A 2012/0123705, to detect possible damage to the at least one measuring tube as early as possible, damage which can bring about an undesired lessening of the accuracy of measurement and/or the operational safety of the respective density measuring device.
In the case of measuring transducers with two measuring tubes, these are most often connected into the particular process line via a distributor piece extending on the inlet side between the measuring tubes and an inlet-side connecting flange as well as via a distributor piece extending on the outlet side between the measuring tubes and an outlet-side connecting flange. In the case of measuring transducers with a single measuring tube, such communicates with the process line most often via a connecting tube opening on the inlet side as well as via a connecting tube opening on the outlet side. Furthermore, measuring transducers with a single measuring tube comprise, in each case, at least one one piece or multipart, for example, tube-, box- or plate-shaped, counteroscillator, which is coupled to the measuring tube at a first coupling zone on the inlet side and is coupled to the measuring tube at a second coupling zone on the outlet side, and which during operation essentially rests or oscillates oppositely to the measuring tube. The inner part of the measuring transducer formed by means of measuring tube and counteroscillator is most often held alone by means of the two connecting tubes, via which the measuring tube communicates during operation with the process line, in a protective measuring transducer housing, especially in a manner enabling oscillations of the inner part relative to the measuring transducer housing. In the case of the measuring transducers shown, for example, in U.S. Pat. Nos. A 5,291,792, A 5,796,010, A 5,945,609, B 7,077,014, US-A 2007/0119264, WO-A 01/02 816 and WO-A 99/40 394 with a single, essentially straight measuring tube, the latter and the counteroscillator are, such as quite usual in the case of conventional measuring transducers, oriented essentially coaxially to one another, in that the counteroscillator is embodied as a essentially straight hollow cylinder and is so arranged in the measuring transducer that the measuring tube is at least partially jacketed by the counteroscillator. Especially in the case of application of titanium, tantalum or zirconium, respectively alloys thereof, for the measuring tube, used for the counteroscillator are, most often, comparatively cost effective steel types, such as, for instance, structural steel or free-machining steel.
For actively exciting, respectively maintaining, oscillations of the at least one measuring tube, not least of all also the wanted oscillations, measuring transducers of vibration-type have, additionally, an exciter mechanism formed by means of at least one electromechanical, for example, namely electrodynamic, electrostatic or piezoelectric, oscillation exciter acting during operation differentially on the at least one measuring tube and the, in given cases present, counteroscillator, respectively the, in given cases present, other measuring tube. The oscillation exciter, electrically connected with the mentioned measuring device electronics by means of a pair electrical connecting lines, for example, in the form of connection wires and/or in the form of conductive traces of a flexible circuit board, and operated by an electrical exciter signal generated by the measuring device electronics and correspondingly conditioned, namely at least per se adapted to changing oscillation characteristics of the at least one measuring tube, serves, especially, to transduce an electrical excitation power fed by means of the mentioned exciter signal into a drive force acting at a point of engagement formed by the oscillation exciter on the at least one measuring tube.
The exciter signal is, in such case, especially, so conditioned that the drive force, as a result, has a wanted force component introduced into the measuring tube, namely a periodic force component changing with an excitation frequency corresponding to the wanted frequency and effecting the wanted oscillations. This is typically implemented by providing the mentioned exciter signal with a wanted excitation component, namely a harmonic signal component changing with a signal frequency corresponding to the wanted frequency and having, in comparison with possible additional signal components of other frequencies contained in the exciter signal, a highest signal power.
For the mentioned case, in which resonant oscillations corresponding to the bending oscillation fundamental mode serve as wanted oscillation, respectively the excitation frequency is set exactly to the corresponding resonant frequency, a velocity response of the at least one measuring tube, namely a velocity of the oscillatory movements of the at least one measuring tube time changing with the wanted frequency at the point of engagement, has relative to the wanted force component of the drive force, as is known, no phase shift, consequently the wanted force component of the drive force and the velocity response under resonance condition () lie in phase, respectively under resonance conditions a corresponding phase shift angle between the wanted force component and the velocity response amounts to zero. The exciter signal, in such case, is additionally often also conditioned such that the wanted oscillations have an essentially constant oscillation amplitude, in spite of fluctuating density and/or viscosity. This is typically achieved in the case of density measuring devices of the type being discussed by providing the exciter signal, respectively the wanted excitation component, with an impressed electrical current, namely an electrical current controlled by the measuring device electronics to a predetermined effective value largely independent of possible disturbances, and/or by providing the exciter signal, respectively the wanted excitation component, with an impressed voltage, namely a voltage controlled to a predetermined effective value by the measuring device electronics largely independently of possible disturbances.
Oscillation exciters of usually marketed measuring transducers of the vibration-type are typically constructed in the manner of a type of oscillation coil, namely formed by means of a magnet coil—in the case of measuring transducers with a measuring tube and a counteroscillator coupled therewith most often a magnet coil affixed on the latter—as well as a permanent magnet serving as magnet armature, interacting with the at least one magnet coil, and correspondingly affixed on the measuring tube to be moved. The permanent magnet and the magnet coil are, in such case, usually so oriented that they extend essentially coaxially relative to one another. Additionally, in the case of conventional measuring transducers, the oscillation exciter is most often so embodied and placed that it essentially acts centrally on the at least one measuring tube. Alternatively to an exciter mechanism formed by means of an oscillation exciter acting rather centrally and directly on the measuring tube, it is possible, such as mentioned, among other things, in the above mentioned U.S. Pat. No. 6,092,429, for example, also to use exciter mechanisms formed by means of two oscillation exciters affixed not in the center of the measuring tube, but, instead, rather at the inlet, respectively outlet, sides thereof or, such as, among other things, provided in U.S. Pat. Nos. 6,223,605 or 5,531,126, for example, also exciter mechanisms formed by means of an oscillation exciter acting between the, in given cases present, counteroscillator and the measuring transducer housing. As, among other things, shown in U.S. Pat. No. 6,477,901 or WO-A 00/34748, it is possible alternatively to the aforementioned oscillation exciters of electrodynamic type, however, at times, also to use piezoelectric, seismic or—not least of all also in the case of such a measuring transducer, whose at least one measuring tube has a very small caliber of less than 1 mm—electrostatic oscillation exciters for exciting wanted oscillations.
For registering oscillatory movements of the at least one measuring tube, not least of all also those corresponding to the wanted oscillations, measuring transducers of the type being discussed have, furthermore, at least one oscillation sensor placed on the measuring tube, for example, electrically connected with the measuring device electronics by means of its own pair of electrical connecting lines, and adapted to transduce the oscillatory movements into a oscillation measurement signal representing such and containing a wanted signal component, namely a periodic signal component with a signal frequency corresponding to the wanted frequency, and to provide the oscillation measurement signal to the measuring device electronics, for example, namely a measuring- and operating circuit of the measuring device electronics formed by means of at least one microprocessor, for additional processing. In the case of measuring transducers of usually marketed vibronic density measuring devices, the oscillation sensors are most often, insofar, embodied essentially of equal construction with that of the at least one oscillation exciter, in that they work according to the same principle of action as in the case of an electrodynamic oscillation exciter, for example, thus, in each case, are likewise of electrodynamic type. Accordingly, also the oscillation sensors of such a sensor arrangement are most often likewise, in each case, formed by means of a permanent magnet affixed on the measuring tube and at least one coil-, for example, a coil affixed on the, in given cases present, other measuring tube or on the, in given cases present, counteroscillator—permeated by a magnetic field of the permanent magnet and as a result of the oscillatory movements of the at least one measuring tube supplied at least at times with an induced measurement voltage. However, also optically or also capacitively designed oscillation sensors are equally usual for oscillation measurement, for example, even for the case, in which the oscillation exciter is of electrodynamic type.
The fulfillment of the resonance condition () essential in the case of conventional vibronic density measuring devices for measuring the density can during operation, for example, be recognized by the respective measuring device electronics noting that a phase shift angle between wanted excitation component and wanted signal component has achieved a predetermined phase value, namely one corresponding to the above referenced resonance condition, in the case of which the phase shift angle between the velocity response and the wanted force component of the drive force is zero, and remains, at least for a predetermined interval, namely for a time sufficiently long for measuring the density, essentially constant. In order to implement a highly accurate measuring of the density also for media with a density variable within a broad density measurement range and/or changing quickly with time, consequently to provide a density measuring device with an as high as possible dynamic range, the measuring device electronics of measuring devices of the type being discussed are not least of all also adapted so to condition the exciter signal that the excitation frequency of the wanted force component corresponds during the measuring of the density as exactly as possible to a respective instantaneous resonant frequency, for example, thus that of the mentioned bending oscillation fundamental mode, respectively that the excitation frequency is adjusted as quickly as possible to a possibly changed resonant frequency, for instance, as a result of a fluctuating density and/or a fluctuating temperature of the measuring tube. The adjusting of the wanted force component by means of the measuring device electronics occurs in the case of conventional density measuring devices typically with exploitation of the above referenced resonance condition, in such a manner that by means of the at least one oscillation measurement signal-, for example, namely based on its wanted signal component—as well as by means of the exciter signal-, for example, namely by setting the signal frequency of the wanted excitation component—the excitation frequency of the wanted force component is changed continuously, respectively successively, and, indeed, to the extent that, respectively until, the phase shift angle between wanted excitation component and wanted signal component has achieved the predetermined phase value, for example, thus is approximately zero. Electronic circuits suitable for setting, respectively tracking, the wanted frequency of the respective measuring tube to one of its instantaneous resonant frequencies, —for example, an electronic circuit configured as a phase locked loop (PLL) respectively frequency control methods implemented therewith, are known, per se, to those skilled in the art, for example, from the above mentioned U.S. Pat. No. A 4,801,897, respectively US-A 2010/0005887.
Due to the wanted oscillations of the at least one measuring tube, —not least of all also for the case, in which the wanted oscillations of the at least one measuring tube are bending oscillations—there can, as is known, be induced in the flowing medium Coriolis forces also dependent on the instantaneous mass flow rate. These, in turn, can bring about Coriolis oscillations with wanted frequency superimposed on the wanted oscillations and dependent on the mass flow rate, in such a manner that a travel time-, respectively phase difference, also dependent on the mass flow rate, consequently also usable as a measure for the mass flow measurement, can be detected between inlet-side and outlet-side oscillatory movements of the at least one measuring tube performing wanted oscillations and at the same time flowed-through by the medium. In the case of an at least sectionally curved measuring tube, in the case of which there is selected for the wanted oscillations an oscillation form, in which the measuring tube is caused to move like a pendulum in the manner of a cantilever clamped on one end, the resulting Coriolis oscillations correspond, for example, to that bending oscillation mode-, at times, also referenced as a twist mode-, in which the measuring tube executes rotary oscillations about an imaginary rotary oscillation axis directed perpendicular to the imaginary oscillation axis, while, in contrast, in the case of a straight measuring tube, whose wanted oscillations are embodied as bending oscillations in a single imaginary plane of oscillation, the Coriolis oscillations are, for example, bending oscillations essentially coplanar with the wanted oscillations. For the above already mentioned case, in which the density measuring device should supplementally to the density additionally also ascertain the mass flow rate of the respective medium guided in the measuring transducer, measuring transducers of the type being discussed have for the purpose of the registering both inlet-side as well as also outlet-side oscillatory movements of the at least one measuring tube and for producing at least two electrical oscillation measurement signals influenced by the mass flow rate to be measured, furthermore, most often two or more oscillation sensors spaced from one another along the measuring tube and so embodied and arranged, that the oscillation measurement signals generated therewith and fed to the measuring device electronics have not only, such as already mentioned, in each case, a wanted signal component, but, instead, that additionally also between the wanted signal components of both oscillation measurement signals a travel time-, respectively phase difference, dependent on the mass flow rate is measurable. Alternatively or supplementally to measuring also the mass flow rate supplementally to the measuring of the density, it is—such as already mentioned, respectively shown, among other things, in the above mentioned US-A 2011/0265580—additionally also possible directly to measure by means of such measuring transducer of vibration-type, consequently by means of vibronic density measuring devices formed therewith, supplementally also a viscosity of the through flowing medium, for example, based on an electrical excitation power required for exciting, respectively maintaining, the wanted oscillations, respectively based on a damping of the wanted oscillations ascertained based on the excitation power, and to output such in the form of qualified viscosity measured values.
In the case of vibronic density measuring devices of the type being discussed, the ascertaining of the density occurs, such as already mentioned, typically based on actively excited, resonant oscillations of the at least one measuring tube, especially namely based on a measuring of at least one of its instantaneous resonance frequencies. The respective measuring device electronics of conventional vibronic density measuring devices is accordingly also adapted, based on the wanted signal component won from the at least one oscillation measurement signal generated under resonance condition (), recurringly to ascertain a frequency measured value, which represents the respectively current, wanted frequency, consequently the current resonant frequency of the at least one measuring tube, and thereafter with application of one or more mentioned frequency measured values to generate a, typically, first of all, digital, density measured value representing the density of the respective medium, for example, by the performing of corresponding calculating algorithms by the mentioned microprocessor. Since the oscillation characteristics of the at least one measuring tube, not least of all also its respective resonance frequencies, and, associated therewith, density accuracy of measurement, namely an accuracy of measurement, with which the density can be measured, are, as is known, dependent also on a temperature distribution within the respective tube wall of the at least one measuring tube, typically at least also a measuring tube temperature is taken into consideration in the case of such density measurements. This is perceivable, among others, from the above mentioned U.S. Pat. No. A 4,491,009, WO-A 88/02853, WO-A 98/02725 or WO-A 94/21999. For ascertaining temperature, at least one local temperature of the at least one measuring tube on a surface of the tube wall facing away from its lumen is registered by sensor, typically by means of a platinum-resistance of a resistance thermometer or a thermocouple adhered on the surface and electrically connected with the respective measuring device electronics, and the measuring device electronics is, furthermore, adapted, based on a temperature signal representing the temperature of the at least one measuring tube, during operation recurringly to ascertain a temperature measured value representing a temperature of the tube wall and to use such temperature measured value in the calculating of the density, not least of all for the purpose of lessening cross-sensitivity of the density measuring device to temperature influences. The actual measuring of the density occurs in the case of conventional density measuring devices of the aforementioned type ultimately, once the measuring device electronics has detected fulfillment of the resonance condition, by ascertaining by means of the measuring device electronics based on the wanted signal component, for example, extracted by means of a digital signal filter from the at least one oscillation measurement signal, first of all, at least one frequency measured value representing the resonant frequency serving as wanted frequency and then converting the frequency measured value into the corresponding density measured value, namely instantaneously representing the density. The converting of the frequency into the associated density measured value can occur, for example, by forming a reciprocal of a square of the frequency measured value and combining the same together with a corresponding temperature measured value for the instantaneous temperature of the tube wall using a characteristic line function correspondingly furnished in the measuring device electronics—for example, in the form of a calculation algorithm executed by the mentioned microprocessor.
Further improvement of the accuracy of the density measurement in the case of vibronic density measuring devices of the aforementioned type can additionally also be achieved, such as, among other things, also disclosed in the above mentioned US-A 2004/0123645, US-A 2011/0219872, WO-A 94/21999, WO-A 98/02725, when, for the purpose of correcting possible further dependencies of the resonant frequency on other medium-, respectively flow specific, measured variables, such as, for instance, a mass flow rate of the medium flowing in the at least one measuring tube, respectively a pressure reigning within the medium guided in the at least one measuring tube, and/or for the purpose of correcting for possible changes of measuring transducer specific, oscillation characteristics, for instance, as a result of an additional, at times, also irreversible, deformation of the at least one measuring tube located in the static resting position and caused by changed temperature distribution within the tube wall or caused by (clamping-) forces acting on the at least one measuring tube, respectively therefrom resulting additional mechanical stresses within the measuring transducer, corresponding influencing variables are metrologically registered and correspondingly taken into consideration in the calculating of the density, for instance, by conforming corresponding correction terms with the previously indicated characteristic line function. Mechanical deformations of the at least one measuring tube can, as well as also disclosed in the above mentioned US-A 2011/0219872, be registered, for example, by means of one or more strain sensors mechanically coupled with the measuring tube on its surface facing away from the lumen.
Further investigations have, furthermore, shown that additionally also the damping of the wanted oscillations effected by dissipation of oscillatory energy into heat is another influencing variable, which can influence the resonant frequency serving as wanted frequency to a not directly negligible extent, respectively can likewise represent a certain cross-sensitivity for the density measuring device. Since changes of the damping as well as, associated therewith, changes of the corresponding resonant frequency in the case of intact measuring transducer in considerable measures are also determined by changes of the viscosity of the respective medium to be measured, in such a manner that the particular resonant frequency in the case of increasing viscosity decreases, in spite of density remaining constant, there is an opportunity for correction of such changes of the resonant frequency caused by changes of the damping. This is done, first of all, basically by having the measuring device electronics ascertain the viscosity—for example, such as already mentioned, based on an electrical excitation power required for exciting, respectively maintaining, the wanted oscillations—and represent such in at least one viscosity measured value instantaneously representing such and/or in at least one damping value representing a damping of the wanted oscillations dependent thereon, in order thereafter to ascertain the density measured value with application also of the viscosity measured value, respectively the damping value, as well as a correspondingly expanded characteristic line function, namely a characteristic line function also taking into consideration the change of the resonant frequency effected by changes of the viscosity. A disadvantage of such a correction based on measuring the viscosity of the medium guided in the at least one measuring tube, respectively a damping of the wanted oscillations dependent thereon, is not least of all that the damping not only depends on the viscosity but, instead, to a certain degree additionally also on the actually to be measured, consequently, first of all, unknown, density of the medium. As a result of this, also the density measured values ascertained by applying viscosity-, respectively damping, values generated by means of the measuring device electronics can, in fact, still have considerable, in given cases, even intolerable, measurement errors.